With the development of the Internet, a large amount of multimedia services emerge, and the requirements of people on mobile communication are no longer limited to the telephone and message services. At present, the application service is introduced into the multimedia service, and the feature of the application service is that a plurality of subscribers can receive the same data simultaneously, such as video on demand, TV broadcast, video conference, online education, interactive games, etc.
In order to effectively use the resources of the mobile network, the multimedia broadcast multicast service (MBMS) technology is proposed, and the MBMS is a point-to-multipoint service in which a data source sends data to a plurality of subscribers. Through this service, the share of network resources can be achieved, wherein this share includes the resource share of the mobile core network and the access network, and in particular the share of air interface resources. And the MBMS in 3GPP can not only achieve the multicast and broadcast of pure text messages with low rate but also can achieve the multicast and broadcast of multimedia services with high rate.
The MBMS single frequency network transmission (MBSFN) manner is a technology which can improve the spectrum utilization rate of the MBMS service. In the MBSFN, the adjacent cells send the same radio signals synchronously, and the User Equipment (UE) can regard the signals from different base stations as a multi-path signal. A plurality of cells, which send MBMS service radio signals synchronously, constitute MBSFN synchronization area. The cells in the MBSFN synchronization area, which send the same MBMS service signals synchronously, are referred to as MBSFN cells.
Since the MBMS service is the service oriented to the whole network, the same MBMS service may be established on different nodes in the lower-layer network element. In particular, the upper-layer network element sends data stream to the lower-layer network element, wherein this data stream includes a plurality of data bursts and each data burst includes a plurality of data packets. Currently, the MBMS service synchronization of a plurality of network elements among cells is realized through using the following method. FIG. 1 is a block diagram of the logical structure of the upper-layer network element and the lower-layer network element. It can be seen that one upper-layer network element is connected to a plurality of lower-layer network elements to interact signaling. FIG. 2 is a flowchart of a method for realizing the MBMS synchronization according to the prior art. As shown in FIG. 2, it includes the following processing:
Step 202: the upper-layer network element sends MBMS service data packet(s) to each lower-layer network element, wherein this data packet carries timestamp information, data packet sequence number information, accumulated data packet length information, etc. As to one or more successive data packets which need to perform concatenation process of the Radio Link Control (RLC) protocol layer, the upper-layer network element marks (indentifies) the same timestamp information for the one or more data packets, and the one or more data packets which are identified with the same timestamp constitute a data burst, in which one data burst can be a set of successive packets and can also be a single packet;
Step 204: as to each lower-layer network element, it receives the above mentioned MBMS service data packet, wherein RLC protocol layer concatenation process is performed on the data packets in the same data burst and the RLC concatenation is not performed on the packets in different data bursts;
Step 206: the lower-layer network element starts to process the packets in the same data burst according to their sequence numbers at the moment indicated by the identified timestamp;
Step 208: the lower-layer network element detects whether there is packet(s) lost and the number of the lost packets according to the data packet sequence number information in each data packet; and
Step 210: the lower-layer network element detects the accumulated length of the lost data packet according to the accumulated data packet length information carried by each data packet, and constructs a virtual data packet.
The MBMS service packets sent to each lower-layer network element by the upper-layer network element are completely identical, so each lower-layer network element can perform totally consistent process to achieve the synchronous sending of the MBMS service among cells of individual lower-layer network elements.
By virtue of the above method, when detecting packet(s) being lost, the lower-layer network element can construct virtue data packet(s) according to the number and total length of the lost packet(s), wherein the number and total length of these virtual packets are consistent with the number and total length of the lost packets. After the construction of virtual packets, the lower-layer network element adds the constructed virtual packet(s) into the user plane protocol process, as if the lost packets were not lost. However, the lower-layer network element does not send user plane data blocks (RLC PDU or MAC PDU) which contain virtual data packets, to ensure that the process on the subsequent packets is the same as other lower-layer network elements and avoid the radio interference with the adjacent cells caused by the inconsistency between the virtual packets and real packets.
It needs to note that the above one upper-layer network element and one or more lower-layer network elements can be the same network element in terms of physical function, and can also be different network elements. The classifications of the upper-layer network element and the lower-layer network element are divided logically to cooperate to complete the function of service synchronization. That is, a plurality of same or different physical network elements are divided into an upper-layer network element and one or more lower-layer network elements according to the logical function. These network elements cooperate to achieve sending MBMS service by way of combining a plurality of cells in the lower-layer network element cells.
In the above synchronization method of MBMS service among cells of the plurality of network elements, each lower-layer network element cell independently completes the radio link control protocol (RLC) process on the MBMS service data packets, which in particular includes: allocating a RLC sequence number, RLC segmentation and concatenation. In normal situation, each lower-layer network element maintains the current RLC sequence number and performs the above RLC process according to the currently received packets. Since the initial RLC sequence number of each lower-layer network element can be kept synchronous by configuration, each lower-layer network element can maintain the consistency of RLC sequence number allocation during the process of data processing.
However, in the existing synchronization method of MBMS service among cells of a plurality of network elements, there is the problem that is: after one certain lower-layer network element restarts due to management reason or abnormal situation, this lower-layer network element is unable to determine a correct radio link control protocol sequence number, which causes it is incapable of maintaining RLC sequence number synchronization with other lower-layer network elements which do not restart.